The choroid plexus (CP) forms a major interface of the blood- cerebrospinal fluid (CSF) barrier and is responsible for the secretion of a large fraction of the CSF. Recent work has demonstrated important biosynthetic activities of the CP, suggesting that the CP also plays a vital role in determining the macromolecular content of the CSF. We (and others) have demonstrated the specific and abundant synthesis of transthyretin (TTR, prealbumin), a transport protein for thyroxine and retinol, within the CP epithelium. We have also described the synthesis of insulin-like growth factor II (IGF-II), a fetal mitogen bearing structural similarity to proinsulin, by the CP epithelium and leptomeningeal membranes of the adult rat brain. The presence of mRNA for transferrin, ceruloplasmin and the beta-preamylod protein have been reported, and we have shown, by immunohistochemistry, the presence of several other specialized enzymes and plasma proteins within the CP. Taken together, these data suggest that the CP is an important determinant of the composition of the external milieu of the brain. I propose to pursue three major lines of investigation deriving from our previous work: 1) Gene expression in the mature CP. We will expand the study of the biosynthetic repertoire of the CP, thereby generating a panel of CP-expressed gene probes. In selected cases (eg. TTR and IGF- II), we will begin to investigate the physiological significance of their intra-CNS synthesis. 2) Developmental studies of the CP. The panel of CP DNA probes will be used to study patterns of gene expression in the developing CP. A major focus of this proposal is an attempt to develop an in-vitro system for studying the morphogenesis of the CP. The specificity of our available probes suggests a novel approach to this question: by co-culturing ventricular ependyma and leptomeninges and attempting to simulate differentiation of the CP, as assayed by the onset of de novo TTR synthesis. 3) Neoplasia of the CP. The CP gene panel will be employed to study gene expression in tumors of the CP in humans and animals. Human CP papillomas and the SV40/transgenic mouse model of multifocal CP papillomas will be studied for patterns of up- or down- regulated genes. Where relevant (eg. IGF-II), other brain tumors will also be studied. In all of the above, as in our previous work, classic molecular gentic techniques will be complemented by a strong emphasis on morphological techniques, including immunohistochemistry and in situ hybridization.